Satellite Positioning Systems (“SPS”) and other positioning technologies have been used for locating persons, things, etc. by, for example, acquiring signals transmitted by transmitters located at fixed locations. In particular implementations, positioning technologies on a mobile device may obtain a “position fix” from time to time indicating an estimate of a location of the mobile device. In combination with position fixes obtained using positioning technologies, some location based applications employ a “geofence” bounding a region of interest to detect entries into or exits from the region of interest. A geofence may comprise a virtual perimeter on a geographic area using a location-based service, so that a notification may be generated in response to the monitored device entering or exiting the area. Detection of a geofence breach may involve monitoring the position of a mobile device and detecting events triggered by the monitored position crossing the geofence.